This invention relates to force responsive transducers and particularly to differential capacitive transducers that are capable of high linearity and precision operation, but at the same time are amenable to low cost fabrication techniques.
Various capacitive transducers are well known in the art. Many of these transducers include two opposed surfaces, each including an electrode, with each surface being sealed about its periphery, typically with a rim facing in the same direction as the electrode. Pressure is applied to the interior or exterior of the cavity thus formed, with one or more of the electrode-bearing surfaces comprising a deflectable diaphragm. Examples of variations of this technique are shown by the patents to Polye U.S. Pat. Nos. 3,634,727 and 3,858,097, the patent to Birchall 3,952,234 and the patent to Johnston 3,808,480. A high fidelity pressure transducer is revealed in the patent to Dias et al, 4,064,550, in which both the body and the deflectable diaphragm consist of quartz, which is too expensive for general application. All of these transducers present certain problems when one attempts to meet conflicting requirements as to cost, precision, size and interchangeability of transducers in modern systems applications. For example, the spacing between facing electrodes is determined in these devices by the peripheral rim, which must thus be closely controlled but can also interfere with the precise deposition of thin or thick film electrodes. This is only one factor to consider, however, inasmuch as it is desirable to achieve linearity and dynamic range that are at least comparable to existing devices while still having a low cost, mass producible design. In pressure transducers used with computerized carburetion or fuel injection systems, for example, the costs must be at least an order of magnitude lower than that which can be tolerated with instrumentation applications, and yet the linearity and range may have to be comparable. Further, it is highly desirable if not essential that the transducers be interchangeable without adjustment in the field. This means in turn that such interchangeability must be achieved on a mass production basis, without substantially raising costs. It is now common practice in the thin and thick film industries, and in the integrated circuit industry, to utilize computerized control and laser trimming to effect adjustments in electrical parameters so that operative characteristics can be set at predetermined nominal values. Using conventional single ended capacitive designs, however, it is extremely difficult to adjust both the zero setting and the range of a transducer arranged in a circuit to provide a varying frequency output.
Differential capacitor transducers are known, as evidenced by the previously referenced patent to Birchall. In the prior art differential effects are achieved using more than two separated electrode-bearing surfaces. This multiplicity of surfaces requires precision alignment, is expensive to manufacture and is seldom interchangeable. Also known is the patent to Lee et al, U.S. Pat. No. 3,859,575. In this construction, the flexible diaphragm comprises the outer circumferential portion of the end cap of a hollow metal cylinder which can be threaded into a base structure. A central post in the end cap provides a support for a spaced plate which has electrodes in opposition to the metal diaphragm which acts as an electrode. The interior of the hollow cylinder provides the internal volume that can contain the variable pressure fluid. This constuction is extremely difficult to machine to shape, and is inordinately expensive for high volume applications. It does provide a different approach to the problem of controlling the spacing between the capacitive means because it departs from the concept of using a fixed reference surface.
Improved transducers using capacitance effects can be utilized in various types of force, pressure displacement and load measuring applications. There is in general a need for such devices which are extremely small in size in comparison to present transducers, capable of withstanding shock and vibrations, and substantially insensitive to temperature variations. Transducers used in the engine compartment of a vehicle, for example, must be as small in size as possible but at the same time must be able to withstand the mechanical forces, temperature variations and other environmental conditions encountered while operating without deterioration over a long time span. Obviously, it is not desirable that the transducer have to be finely adjusted either during initial installation or in replacement.
The vehicular application is a good example of a broader problem encountered with transducers, because of the increasing usage of microprocessors in other advanced analog and digital systems which receive transducer outputs and utilize these in effecting computations or controls. The essential problem with the transducer is to provide a signal that varies linearly with the input parameter variation, in such form that it may be utilized directly by the processor. For a digital system, the common approach is to employ an analog-to-digital converter for this purpose, but this adds an undesirable increment of cost that should be avoided if possible.